drm/i915: Extract hsw_ddi_{enable,disable}_clock()

Yank out the HSW/BDW code from intel_ddi_clk_{select,disable}()
and put it into the new encoder .{enable,disable}_clock() vfuncs.

v2: s/dev_priv/i915/ (Lucas)
v3: Deal with FDI

Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com> #v2
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210205214634.19341-4-ville.syrjala@linux.intel.com